Yeah, there is a product called Extend... it's designed to stop and prevent rust. Go to a local autozone or NAPA or something... I just did this on my car this summer. Had rust (though the metal) on the rear wheelwells, and we sanded it, sprayed it, and bondo/fiberglassed it (then alot of sanding, and then paint), and looks good as new.

I don't have a before pic, but here's a few after pics...




Good luck, if you need any help or anything, let me know. This process took about a week of on-and-off work... an hour here, an hour there, etc. If you really worked on it all day, you could probably have it done in less than 2 days.